Water Quality Index Calculators

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The Water Quality Index (WQI) is a composite score that translates multiple individual water quality measurements into a single number expressing the overall quality of a water body for a specific use (drinking, recreation, aquatic life). By aggregating parameters such as pH, dissolved oxygen, turbidity, conductivity, nitrates, and coliform bacteria into one index, WQI makes complex water quality data accessible to managers, policymakers, and the public. Various WQI systems exist — including NSF-WQI, Canadian WQI, and the WHO drinking water assessment — each selecting parameters and weighting them based on intended water use.

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What Is the Water Quality Index?

WQI is a mathematical tool that aggregates multiple water quality parameters into a single score, typically on a 0–100 scale, where higher scores indicate better quality.

NSF Water Quality Index — Parameters and Weights

The National Sanitation Foundation (NSF) WQI uses nine parameters:

ParameterWeight
Dissolved oxygen (%)0.17
Fecal coliform (CFU/100 mL)0.16
pH0.11
BOD5 (mg/L)0.11
Temperature change (°C)0.10
Total phosphate (mg/L)0.10
Nitrates (mg/L)0.10
Turbidity (NTU)0.08
Total solids (mg/L)0.07

WQI Calculation

For each parameter:

  1. Measure the raw value
  2. Convert to a sub-index score (qi, 0–100) using parameter-specific conversion curves
  3. Multiply by weight (wi): qi × wi
  4. Sum all weighted sub-indices: WQI = Σ(qi × wi)

WQI Classification (NSF)

  • 91–100: Excellent
  • 71–90: Good
  • 51–70: Medium
  • 26–50: Bad
  • 0–25: Very bad

Key Water Quality Parameters

  • Dissolved oxygen (DO): Low DO indicates organic pollution; critical for aquatic life. Target: ≥7 mg/L for coldwater fish
  • Biochemical oxygen demand (BOD): Organic pollution load; WHO: <3 mg/L for clean water
  • pH: Optimal 6.5–8.5 for drinking water and most aquatic life
  • Fecal coliform: Indicator of sewage contamination; 0 CFU/100 mL required for drinking water
  • Turbidity: Suspended particles; <1 NTU for drinking water

Glossary

Water Quality Index (WQI)
A composite 0–100 score aggregating multiple water quality parameters (pH, DO, BOD, fecal coliform, turbidity, etc.) into a single quality indicator. Higher scores = better quality. Used for monitoring, management, and public communication.
Dissolved Oxygen (DO)
The amount of oxygen dissolved in water, measured in mg/L or % saturation. Critical for aquatic life — <5 mg/L stresses fish; <2 mg/L causes mortality. The highest-weighted parameter in the NSF WQI.
Biochemical Oxygen Demand (BOD)
The amount of oxygen consumed by microorganisms decomposing organic matter in water over 5 days at 20°C (BOD5). Indicates organic pollution load. Clean water: <3 mg/L; heavily polluted: >10 mg/L.

Frequently Asked Questions

WQI is a composite score (0–100) that aggregates multiple water quality parameters into one number. For each parameter: (1) measure the value; (2) convert to a sub-index score (0–100) using conversion curves reflecting the parameter's importance to water quality; (3) multiply by the parameter's assigned weight. WQI = sum of all (sub-index × weight). Higher scores mean better quality. The NSF WQI uses 9 parameters with weights summing to 1.0.

The NSF WQI includes: dissolved oxygen (weight 0.17), fecal coliform (0.16), pH (0.11), biochemical oxygen demand BOD5 (0.11), temperature deviation (0.10), total phosphate (0.10), nitrates (0.10), turbidity (0.08), and total solids (0.07). Each reflects a different dimension of water quality — microbial safety, oxygen content, nutrients, suspended matter, and acidity.

Using the NSF WQI scale: 91–100 = Excellent (pristine); 71–90 = Good (suitable for drinking with standard treatment); 51–70 = Medium (treatment needed; aquatic life may be stressed); 26–50 = Bad (limited aquatic life; not suitable for recreation); 0–25 = Very bad (heavy pollution; dangerous for contact). These categories help communicate water quality status to non-scientists and guide management decisions.

Dissolved oxygen (DO) has the highest weight (0.17) in the NSF WQI because it directly determines the capacity of water to support aquatic life and reflects overall ecosystem health. DO below 5 mg/L stresses fish; below 2 mg/L causes mass mortality. DO is depleted by organic pollution (BOD) and eutrophication. It integrates the effects of temperature, organic load, photosynthesis, and aeration — making it one of the most informative single indicators of water quality.